Pyroelectric photoconductive elements and method of charging same

ABSTRACT

The recording element comprises an electrically conductive substrate and a layer of a photoconductive, pyroelectric compound on the substrate. The layer comprises one or more wafers of the pyroelectric compound, each wafer being similarly oriented on the substrate so that its polar c-axis is transverse to the opposite large surfaces of the layer. The novel electrophotographic recording element is charged by merely changing the temperature of the layer a few degrees.

United States Patent 1 1 Kiess 1 1 Jan. 30, 1973 PYROELECTRICPHOTOCONDUCTIVE ELEMENTS AND METHOD OF CHARGING SAME Inventor: HelinutGustav Kiess, l-lightstown,

Assignee: RCA Corporation Filed: Aug. 31,1970

Appl. No.: 68,067

us. (:1. "9.6/1.5, 96/1 R, 96/1 c,

96/] .8, 317/262 A, 317/262 AE 1111. c1. ..G03g 5/02, 003 13/02 Field ofSearch ..96/l, 1.5 1.8;23/147, 148

References Cited UNITED STATES PATENTS 5/1971 Williams ..96/1 .8 1/1967Goffe ..96/1.8

OTHER PUBLICATIONS Berlincourt et al., Electroplastic Properties of theSulfides, Selenides, and Tellurides of Zinc and Cadmium, Phys. Rev.,Vol. 129, No. 3, Feb. 1963, pp.

Minkus, Temperature Dependence of the Pyroelec-' tric effect in CadmiumSulfide, Phys. Rev., Vol. 138, No. 4A, May, 1965, pp. 1277l287.

Heiland 'et al., Pyroelectricity of Zinc Oxide," Solid State Comm,Vol.4, 1966, pp. 353-356.

Grigas, Pyroelectric effect in SBSI," C'.A., Vol. 69, 1968, pp. 910828.I 5

Primary ExaminerGeorge F. Lesmes Assistant ExaminerRoland E. Martin, Jr.Att0rneyGlenn H. Bruestle [57] ABSTRACT 11 Claims, 6 Drawing FiguresLPATENTEDJMOIQB 3.713.822

. INVENTOR. Helmut 6. Kiess v ATTORNEY P YROELECTRIC PI-IOTOCONDUCTIVEELEMENTS AND METHOD OF CHARGING SAME BACKGROUND OF THE INVENTION Thisinvention relates generally to an electrophotographic recording elementand a method of charging same. More particularly, the invention relatesto a novel electrophotographic plate of a pyroelectric compound and anovel method of charging the electrophotographic plate by changing itstemperature. The novel electrophotographic plate and method of chargingit are particularly useful in an electrophotographic process wherein animage developed on the plate is to be transferred from the plate to atransfer sheet.

It has been proposed to charge an electrophotographic recording element,comprising a photoconduc- I tive layer, by subjecting thephotoconductive layer to a corona discharge from a corona dischargedevice. While this prior-art method is the most conventional one forcharging an electrophotographic recording element, it requires arelatively expensive, high-voltage power supply, capable of generatingat least 5,000 volts, and a corona discharge device that must becarefully shielded and insulated to protect an operator or maintenancepersonnel from electric shock. Also, unless the high-voltage equipmentis suitably shielded, it can cause annoying radio and televisioninterference.

The novel electrophotographic recording element and method of chargingit make it possible to carry out an electrophotographic process withouta high voltage power supply, thereby eliminating the inherentdisadvantage thereof.

SUMMARY OF THE INVENTION The novel electrophotographic recording elementcomprises a layer of a photoconductive pyroelectric material on arelatively electrically conductive substrate and in electrical contacttherewith. The pyroelectric material of the layer is oriented so as toproduce electrostatic charges on the opposite surfaces of the layer whenthe temperature of the layer is changed.

The novel method of charging the novel recording element compriseschanging the temperature of the layer to produce an electrostatic chargebetween the opposite surfaces of the layer.

The novel recording element and method employ the principles ofpyroelectricity and obviate the need for relatively expensivehigh-voltage power supplies, corona discharge devices, and the necessarysafety precautions required therefore. The novel method is alsorelatively free from conditions that cause interference with radio andtelevision reception.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of ahexagonal, singlecrystal structure of a photoconductive, pyroelectricmaterial, zinc oxide," exhibiting itspolar c-axis;

3 FIG. 2 is a fragmentary plan view of one embodiment of anovelelectrophotographic recording element in the form of a plate, employingelemental bodies of the single-crystal material illustrated in FIG. 1;

FIG. 3 is a fragmentary cross-sectional view of the electrophotographicplate shown in FIG. 2, taken along the line 3-3, and viewed in thedirection indicated by the arrows;

FIG; 4 is a side elevational view of the novel recording element andmeans, illustrated symbolically, for charging it in accordance with thenovel method; and,

FIGS. 5 and 6 are side elevational views of the novelelectrophotographic plate in different steps of forming anelectrophotographic image thereon.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. I of thedrawing, there is shown a single-crystal, photoconductive, pyroelectricmaterial, such as a compound 10, of the type used to make the novelelectrophotographic recording element. The compound 10 may be asingle-crystal, hexagonal structure of zinc oxide, for example, havingopposite polar (0001) surfaces 12 and 14 that are different from eachother with respect to the direction of a c-axis 16, the surface 14 beingdesignated as the Zn surface and the surface 12 being designated as the0 surface. While the novel recording element will be described withrespect to the pyroelectric compound 10 of zinc oxide, it is within thecontemplation of the present invention to use other pyroelectriccompounds, such as CdS, SbSI, or CdSe, for example.

If the temperature of the pyroelectric compound 10 is changed a voltageis produced between the opposite surfaces 12 and 14 that are transverseto the c-axis 16. For example, if the temperature of the compound I0 israised about 5C, the surface 12 acquires a negative electrostatic chargewith respect to the surface 14, the surface 14 acquiring a positiveelectrostatic charge. The polarities of the electrostatic charges on thesurfaces 12 and 14 depend upon whether the c-axis 16 is pointingupwardly, in the direction of the arrow 18, or downwardly in thedirection 20 with respect to the surfaces 12 and 14, looking at FIG. 1.The amplitude of the voltage produced by the change in temperature isproportional to the thickness of the compound 10 between the oppositesurfaces 12 and 14.

One method of determining the polarities of the electrostatic chargesproduced on the faces 12 and 14 of the pyroelectric compound 10 is toetch one of the prism (1010) faces 22 with 40 percent hydrofluoric acid.A plurality of etch pits 24 (only one shown in FIG. 1) are producedwhich can indicate the direction of the polar c-axis. Each etch pit 24has an arrowhead-type shape that points in the positive direction of thec-axis. Hence, the etch pit 24, in FIG. 1, indicates that the polar axis16 is positive in the direction of the arrow 20 and negative in thedirection of the arrow 18.

The pyroelectric compound 10 of zinc oxide may be grown by any suitablemethod known in the art or it may be purchased from commercial vendors,as from Litton, Inc., Airtron Division, Morris Plains, New Jersey; orfrom the Dielectric Materials and System Division of The 3M Company, St.Paul, Minnesota. The zinc oxide crystal compound 10 may be purchasedeither in the uncut crystal form or in the form of wafers 28 cut fromthe crystal, for example, along the dashed line 26'parallel to the faces12 and 14. Uncut crystals are commercially available in diameters of 1mmthrough 5 mm and in varying lengths of 2 mm to 10 mm, depending on theirdiameter. Square wafers of zinc oxide compound 10 having a side of 10 mmand a thickness of at least about 5 mm are also available commercially.The zinc oxide compound 10 is preferably doped with a suitable dopant,such as copper (concentration of about l-l0 cm' or lithium(concentration of about 5.l0 cm' to compensate the zinc oxide withP-type carriers so as to provide it with a suitable resistivity. Whendoped, the dark resistivity of wafers 28 of the compound 10 is at leastl0ohm cm, and usually much higher. The doped single-crystal, zinc oxidecompound 10 possesses an illuminated resistivity of approximately 10 ohmcm.

Referring now to FIGS. 2 and 3, there is shown a novel recording elementin the form of an electrophotographic plate 30 suitable for use in anelectrophotographic process for reproducing images. The plate 30comprises an electrically conductive substrate 32, such as a sheet ofaluminum, copper, or stainless steel, and a photoconductive layer 34 ofone or more wafers 28 of the pyroelectric compound 10. As shown in FIGS.2 and 3, the photoconductive layer 34 is a composite of a plurality ofthe pyroelectric elemental bodies, such as wafers 28. The wafers 28 areadhered to the substrate 32 by any suitable electrically conductivepaste 36 or solder. While the wafers 28 of zinc oxide,illustrated inFIGS. 2 and 3, are shown as being hexagonal, they may be in othershapes, such as rectangular or square. The layer 34, however, shouldhave its opposite (large) major surfaces 38 and 40 substantiallycontinuous; that is, the wafers 28 should abut each other as closely aspossible so as to provide substantially continuous, smooth, oppositemajor surfaces 38 and 40. The lower surfaces of the wafers 28 that arein electrical contact with the substrate 32 may be coated with a verythin coating of indium so as to make a good electrical contact with thesubstrate 32 through the conductive paste 36. To provide a layer 24 of acomposite of wafers 28, each wafer 28 should preferably have a thicknessof at least about 0.5 mm, one major surface area of at least 2.5 mm, anda resistivity of at least ohm-cm in the dark.

All of the wafers 28 of the electrophotographic plate 30 are oriented sothat their c-axes are transverse to, and preferably perpendicular to,the opposite major surfaces 38 and 40 of the lay 34. Since thephotoconductor of zinc oxide in most electrophotographic processes ischarged negatively, the zinc oxide wafers 28 of the compound 10 aredisposed in the layer 34 so that their c-axes point outwardly, as in thedirection 18 of FIG. 1, from the upper major surface 38 of the layer 34.Thus, a negative electrostatic charge is produced upon the upper majorsurface 38 of the layer 34 when the temperature of the layer 34 israised, as will be explained hereinafter. Conversely, the upper majorsurface 38 of the photoconductive layer 34 will exhibit a positiveelectrostatic charge if the temperature of the layer 34 is suddenlylowered from its ambient.

The upper surface 38 of the photoconductive layer 34 is preferablycoated with a monolayer of one or more sensitizing dyes, such asfluorescence, cyanine dyes, rose bengal, or erythrosin, for example, tomake the photoconductive layer responsive to visible light. In theabsence of such dye sensitization, the photoconductive layer 34 of zincoxide wafers 28 can be exposed by ultraviolet light to which it isparticularly sensitive.

Referring now to FIG. 4, means are shown to electrostatically charge thenovel electrophotographic plate 30 by changing its temperature, indarkness, in accordance with the novel method. To this end, a heater 42,such as a hot plate, is disposed adjacent to the electrophotographicplate 30. The heater is connected in series with a suitable voltagesource 44 and a switch 46. Thus, to charge the electrophotographic plate30, the switch 46 is closed to energize the heater 42. The voltagedeveloped pyroelectrically across the photoconductive layer 34 of theelectrophotographic plate 30, in darkness, is proportional to thethickness of the layer 34; that is, the thickness of the wafers 28, andto the change in the temperature of the layer 34. A thickness of thephotoconductive layer 34 of about 0.7 mm will provide a voltagethereacross of about 250 volts when the temperature of the layer 34 israised about 5C. If the c-axes of the wafers 28 are pointing outwardlyfrom the upper surface 38 of the photoconductive layer 34, the uppersurface 38 of the photoconductive layer 34, the upper surface 38 ischarged negatively and the lower surface 40 is charged positively, asindicated symbolically in FIG. 4. If the c-axes of the wafers 28 werepointing downwardly from the upper surface 38, the charges on thesurfaces 38 and 40 of the photoconductive layer 34 would be reversed.

A change in temperature of the photoconductive layer 34 of between about5C and 30C from the ambient has been found suitable to charging thenovel electrophotographic plate 30 for processing in anelectrophotographic process. A change in temperature of less then'5C maynot develop a sufficient charge on a very thin layer 34, while a changein temperature of more than 30C may render the layer 34 too conductiveto retain the charge for a desired period.

Although a preferred method of charging the novel electrophotographiclate 30 is by raising its temperature from the ambient, it is within thecontemplation of the novel method to charge the electrophotographicplate 30 by cooling it between about 5C and 30C from the ambient. Thismay be accomplished by blowing refrigerated air onto the surface 38 ofthe layer 34, from any suitable source, through a plurality of jets 47disposed above the photoconductive layer 34, as shown in FIG. 4. Anyother means of cooling, such as by Peltier elements, for example, mayalso be used.

The charged electrophotographic plate 30 can now be exposed to a lightimage to be reproduced, whereby to discharge the electrophotographicplate 30 selectively and to provide an electrostatic latent image on thephotoconductive layer 34. This operation in the electrophotographicprocess is illustrated in FIG. 5. A transparency 48 of the image to bereproduced, as a frame of a motion picture film 50, for example, isprojected through a light projector 52 to the upper surface 38 of acharged (previously heated) photoconductive layer 34. The exposure stepshould be carried out as soon as possible after the charging step toprevent a loss of the charge with a change in temperature. Those porbyany suitable developing means known in the electrophotographic art. Forexample, the surface 38 may I be brushed with a magnetic brushcomprising a magnet 54 and a triboelectric mixture 56 of iron and tonerparticles, as taught in U.S. Pat. No. 2,874,063, issued on February 17,1959, to H. G. .Greig for Electrostatic Printing. Thetoner particles 58are positive. in the triboelectric mixture 56 and are attracted to thenegative charges of the latent electrostatic image, thereby developingthe latent image, as shown in FIG. 6., The

developed image may be fixed, as by heating, on the surface 38 of thephotoconductive layer 34, or, if desired, the unfixed image may betransferred to a transfer sheet by any suitable means and methods wellknown in the art for transferring an unfixed image from anelectrophotographic plate to a transfer sheet.

Although the improved electrophotographic plate has been shown anddescribed as comprising singlecrystal, photoconductive, pyroelectricwafers of zinc oxide which is white, the photoconductive layer of theplate may comprise other pyroelectric, photoconductive compounds. Whilesome of the pyroelectric compounds, such as CdS, SbSI, or CdSe, are notwhite, they are quite suitable for producing unfixed images on the novelelectrophotographic plate for transfer purposes to transfer surfaces.The novel method of charging the novel electrophotographic plateeliminates the need of the high voltage power supply of the prior artand the attendant disadvantages inherent herein. The dimensions of theembodiments of the electrophotographic plate and conditions of themethod of charging it described herein are merely illustrative and arenot intended to be considered in a limiting sense.

I claim: 1. An electrophotographic recording element comprising;

a substrate of electrically conductive material, and a layer ofphotoconductive, pyroelectric material comprising a composite of similarcrystalline elemental bodies, said layer being in electrical contactwith said substrate, and said elemental bodies of said material beingsimilarly oriented with respect to their crystal axes to produceelectrostatic charges of the same polarity on an exposed surface of saidrecording element when the temperature of said layer is changed. 2. Anelectrophotographic recording element as described in claim 1, whereinsaid material is a compound selected from the group consisting ofsingle-crystal ZnO, CdS, CdSe, and

SbSI, and

5. An electrophotographic recording element as described in claim 1,wherein said material is single-crystal zinc oxide, said layer comprisesone or more bodies of said single-crystal zinc oxide, and each of saidbodies has a c-axis pointing toward said substrate.

6. An electrophotographic recording element as described in claim 4,wherein each of said wafers has a thickness of at least 0.5 mm, a largesurface with an area of at least 2.5 mm, and a resistivity of at leastlO ohm-cm in the dark. 7. In an electrophotographic method of the typewherein a photoconductive layer of a recording element is sequentiallyelectrostatically charged, selectively discharged with a light image toprovide an electrostatic latent image, and said latent image isdeveloped with an electroscopic toner to provide a visible image on saidlayer, the improvement of charging said layer comprises one or moreelemental bodies of said compound. 3. An electrophotographic recordingelement as described in claim 3, wherein each of said elemental bodieshas a c-axis, and said caxes extend in the same direction and aresubstantially perpendicular to said surface of said layer. 4. Anelectrophotographic recording element as described in claim 1, whereinsaid material is single-crystal ZnO, said layer comprises a plurality ofwafers of said single-crystal ZnO, and each of said wafers has a c-axisextending outwardly from said exposed surface and said layer.

said recording element comprising the steps of:

providing said photoconductive layer in theform of one or more similarcrystalline elemental bodies of a photoconductive, pyroelectric materialon an electrically conductive substrate, each of said elemental bodiesbeing similarly oriented with respect to their crystal axes to produceelectrostatic charges of opposite polarity on the opposite largesurfaces, respectively, of said layer when the temperature of said layeris changed, and changing the temperature of said layer to produce saidelectrostatic charges, whereby to charge said layer. 8. In anelectrophotographic method of the type described in claim 7, wherein thestep of changing the temperature of said layer to produce saidelectrostatic charges comprises raising the temperature of said layerbetween 5C and 30C. 9. In an electrophotographic method of the typedescribed in claim 7, wherein the step of changing the temperature ofsaid layer to produce said electrostatic charges comprises lowering thetemperature of said layer between 5C and 30C. 10.In anelectrophotographic method of the type described in claim 7, wherein thestep of providing said photoconductive layer comprises providing a layerof one or-more wafers of single-crystal, photoconductive zinc oxide onan electrically conductive substrate, each of said wafers of zinc oxidehaving a c-axis that is pointing in the same direction and orientedsubstantially perpendicularly to opposite large surfaces of said layer,and the step of changing the temperature of said layer comprises heatingor cooling said layer at least about 5C. 11. In an electrophotographicmethod of the type describedin claim 10, wherein said layer has onesurface in electrical contact with anelectrically conductive substrate,and said c-axes of said wafers of zinc oxide point outwardly from theopposite large surface of said layer.

t t l t UNITED STATES PATENT oFFIcE CETIFICATE 0F Patent No. 3,713 ,822Dated January 30 1973 Inv t Helmut Gustav Kiess It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Claim 3, Col. 5, line 55 "Claim 3" should be Claim 1-- Claim 4, Col, 5,line 65 Cancel "and" and insert -of- Column 4, line '36 Cancel "late"and insert --plate Column 5, line 29 Cancel "herein" and insertthereinfiigned and Scaled this twenty-seventh D ay OF April 1 9 76[SEAL] A ttest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer (mnmissinncr oj'lulenlsand Trademarks

1. An electrophotographic recording element comprising; a substrate ofelectrically conductive material, and a layer of photoconductive,pyroelectric material comprising a composite of similar crystallineelemental bodies, said layer being in electrical contact with saidsubstrate, and said elemental bodies of said material being similarlyoriented with respect to their crystal axes to produce electrostaticcharges of the same polarity on an exposed surface of said recordingelement when the temperature of said layer is changed.
 2. Anelectrophotographic recording element as described in claim 1, whereinsaid material is a compound selected from the group consisting ofsingle-crystal ZnO, CdS, CdSe, and SbSI, and said layer comprises one ormore elemental bodies of said compound.
 3. An electrophotographicrecording element as described in claim 3, wherein each of saidelemental bodies has a c-axis, and said c-axes extend in the samedirection and are substantially perpendicular to said surface of saidlayer.
 4. An electrophotographic recording element as described in claim1, wherein said material is single-crystal ZnO, said layer comprises aplurality of wafers of said single-crystal ZnO, and each of said wafershas a c-axis extending outwardly from said exposed surface and saidlayer.
 5. An electrophotographic recording element as described in claim1, wherein said material is single-crystal zinc oxide, said layercomprises one or more bodies of said single-crystal zinc oxide, and eachof said bodies has a c-axis pointing toward said substrate.
 6. Anelectrophotographic recording element as described in claim 4, whereineach of said wafers has a thickness of at least 0.5 mm, a large surfacewith an area of at least 2.5 mm2, and a resistivity of at least109ohm-cm in the dark.
 7. In an electrophotographic method of the typewherein a photoconductive layer of a recording element is sequentiallyelectrostatically charged, selectively discharged with a light image toprovide an electrostatic latent image, and said latent image isdeveloped with an electroscopic toner to provide a visible image on saidlayer, the improvement of charging said recording element comprising thesteps of: providing said photoconductive layer in the form of one ormore similar crystalline elemental bodies of a photoconductive,pyroelectric material on an electrically conductive substrate, each ofsaid elemental bodies being similarly oriented with respect to theircrystal axes to produce electrostatic charges of opposite polarity onthe opposite large surfaces, respectively, of said layer when thetemperature of said layer is changed, and changing the temperature ofsaid layer to produce said electrostatic charges, whereby to charge saidlayer.
 8. In aN electrophotographic method of the type described inclaim 7, wherein the step of changing the temperature of said layer toproduce said electrostatic charges comprises raising the temperature ofsaid layer between 5*C and 30*C.
 9. In an electrophotographic method ofthe type described in claim 7, wherein the step of changing thetemperature of said layer to produce said electrostatic chargescomprises lowering the temperature of said layer between 5*C and 30*C.10. In an electrophotographic method of the type described in claim 7,wherein the step of providing said photoconductive layer comprisesproviding a layer of one or more wafers of single-crystal,photoconductive zinc oxide on an electrically conductive substrate, eachof said wafers of zinc oxide having a c-axis that is pointing in thesame direction and oriented substantially perpendicularly to oppositelarge surfaces of said layer, and the step of changing the temperatureof said layer comprises heating or cooling said layer at least about5*C.